I looked at the Frontier when I was shopping for a new truck a few months ago. I owned a '97 Nissan Kingcab at the time. I decided to get a new Toyota Tacoma Double Cab instead of the Frontier for these reasons.
The Nissan I had ran good, but I'd had a few little problems with it. The transmission was rough, the check engine light came on and the tech couldn't find a reason for it, it came on again a few weeks later and they still couldn't find a reason, it developed a miss with less than 40,000 miles on it and had to have the plugs and distributor? rotor? cap replaced.
It seemed to be kind of cheaply built. The tailgate wouldn't close unless you pulled the handle out as you closed it and it was a little crooked. The back bumper was lower on one side than the other. I never had any problems with the interior, but again it looked cheap compared to others.
I never got the kind of gas mileage I expected.
In comparing the Frontier Crew Cab and the Tacoma Double Cab, I saw the same sorts of differences. The Tacoma seemed to be better built with better quality materials. The Frontier offered more bells and whistles at a slightly cheaper price, but I just saw the Tacoma as better quality where it counted. I also liked the fact that the Tacoma had the off road package (the skid plate was important and the positive traction option), it had a higher clearance, a bigger bed, a surprisingly smoother ride, more leg room in the backseat area, and a more expensive looking interior.
I've had the Tacoma for a little over 2 months now and I'm really glad I traded the Nissan for it, I'm much happier with it.
Oh yeah, and you know those cool looking rivets around the fender flares on the Frontier? I heard that they're constantly coming loose and getting lost if you have to do any off-road driving at all.
Just my experiences and opinions.